Wave energy commercialization lags well behind wind energy despite the fact that water is several hundred times denser than air and waves remain for days and even weeks after the wind which originally produced them has subsided. Waves, therefore, efficiently store wind kinetic energy at much higher energy densities, typically averaging up to 50 to 100 kw/m of wave front in many northern latitudes.
Hundreds of uniquely different ocean wave energy converters (OWECs) have been proposed over the last century and are described in the patent and commercial literature. Less than a dozen OWEC designs are currently deployed as “commercial proto-types.” Virtually all of these suffer from high cost per average unit of energy capture. This is primarily due to the use of heavy steel construction necessary for severe sea-state survivability combined with (and in part causing) low wave energy capture efficiency. Only about 10% of currently proposed OWEC designs are deployed subsurface where severe sea-state problems are substantially reduced. Most subsurface OWECs are, unfortunately, designed for near shore sea bed deployment. Ocean waves lose substantial energy as they approach shore (due to breaking or reflected wave and bottom and hydrodynamic friction effects). Near shore submerged sea bed OWECs must be deployed at greater depths relative to average wave trough depths due to severe sea-state considerations to avoid breaking wave turbulence, and depth can not be adjusted for the large tidal depth variations found at the higher latitudes where average annual wave heights are greatest. Wave induced subsurface static pressure oscillations diminish more rapidly in shallow water as the depth below waves or swell troughs increases.
Only a few prior art subsurface devices use gas filled or evacuated containers like the present invention, producing container deformation in response to overhead swell and trough induced static pressure changes. None of the prior art subsurface OWECs capture both hydrostatic (heave) and hydrokinetic wave energy (surge or pitch) which represents half of all wave energy. None of these prior art subsurface OWECs enhance or supplement energy capture with overhead floating bodies. All of the prior subsurface deformable container OWECs suffer from high mass (and therefore cost) and low energy capture efficiency (even more cost) usually due to near shore or sea bed deployment and high mass. None of these have the tidal and sea-state depth adjustability of the present invention needed for enhanced energy capture efficiency and severe sea-state survivability. None have the low moving mass (allowing both short wave and long swell energy capture) and the large deformation stroke (relative to wave height) needed for high capture efficiency of the present invention.
At least two prior art devices use two variable volume gas filled containers, working in tandem, to drive a hydraulic turbine or motor. Gardner (U.S. Pat. No. 5,909,060) describes two sea bed deployed gas filled submerged inverted cup shaped open bottom containers laterally spaced at the expected average wavelength. The inverted cups are rigidly attached to each other at the tops by a duct. The cups rise and fall as overhead waves create static pressure differences, alternately increasing and decreasing the gas volume and hence buoyancy in each. The rise of one container and concurrent fall of the other (called an “Archemedes Wave Swing”) is converted into hydraulic work by pumps driven by said swing.
Similarly, Van Den Berg (WO/1997/037123 and FIG. 1) uses two sea bed deployed submerged average wavelength spaced interconnected pistons, sealed to underlying gas filled cylinders by diaphragms. Submerged gas filled accumulators connected to each cylinder allow greater piston travel and hence work. The reciprocating pistons respond to overhead wave induced hydrostatic pressure differences producing pressurized hydraulic fluid flow for hydraulic turbines or motors.
The twin vessel Archemedes Wave Swing (“AWS”) of Gardner (U.S. Pat. No. 5,909,060) later evolved into a single open bottomed vessel (FIG. 2) and then more recently Gardner's licensee, AWS Ocean Energy has disclosed an enclosed gas filled vessel (an inverted rigid massive steel cup sliding over a second upright steel cup) under partial vacuum (FIG. 3). Partial vacuum, allowing increased stroke, is maintained via an undisclosed proprietary “flexible rolling membrane seal” between the two concentric cups. Power is produced by a linear generator (FIG. 2 shown) or hydraulic pump driven by the rigid inverted moving upper cup. An elaborate external frame with rails and rollers, subject to fouling from ocean debris, is required to maintain concentricity and preserve the fragile membrane.
FIG. 4 (Burns U.S. 2008/0019847A1) shows a submerged sea bed mounted gas filled rigid cylindrical container with a rigid circular disc top connected by a small diaphragm seal. The disc top goes up and down in a very short stroke in response to overhead wave induced static pressure changes and drives a hydraulic pump via stroke reducing, force increasing actuation levers. Burns recognizes the stroke and efficiency limitations of using wave induced hydrostatic pressure variations to compress a gas in a submerged container and attempts to overcome same by arranging multiple gas interconnected containers perpendicular to oncoming wave fronts. North (U.S. Pat. No. 6,700,217) describes a similar device. Both are sea bed and near shore mounted and neither is evacuated or surface vented like the present invention to increase stroke and, therefore, efficiency.
FIG. 5 (Meyerand U.S. Pat. No. 4,630,440) uses a pressurized gas filled device which expands and contracts an unreinforced bladder within a fixed volume sea bed deployed rigid container in response to overhead wave induced static pressure changes. Bladder expansion and contraction within the container displaces sea water through a container opening driving a hydraulic turbine as sea water enters and exits the container. Expansion and contraction of the submerged bladder is enhanced via an above surface (shore mounted) diaphragm or bellows. High gas pressure is required to reinflate the submerged bladder against hydrostatic pressure.